Tectonic significance of plutonism in the Thirtymile Range, southern Yukon

Two distinct but undeformed suites of granitic plutons intrude deformed siliciclastic rocks in western Dorsey Terrane. A calc-alkaline hornblende-bearing gabbro to granodiorite stock has been dated at 181.5 Ma (by the Rb/Sr method). The second suite consists of highly evolved late-orogenic granites of the Thirtymile stock and Hake Batholith, which are approximately 100 Ma. The penetrative fabric of the metasedimentary rocks indicates generally eastward-vergent layer-parallel shear. The deformation of the siliciclastic rocks is thus constrained at older than 181 Ma. The absence of resetting of the Rb-Sr isotopic ratios of the Jurassic pluton indicates that the mid-Cretaceous magmatism was emplaced at a shallow crustal depth. Since the Jurassic pluton has both a 'juvenile' Sr isotopic ratio of 0.7045 and chemistry indicative of a largely mantle-derived source, a subduction-related setting for magma generation is likely. The spatial relationship of craton-derived clastic rocks and these plutons requires that subduction had an eastward polarity.